A conventional media filter has a bed of media laid over a drainage system. The most common media filter in municipal drinking water plants is a gravity sand filter, also called a rapid sand filter, in which the media is sand. The drainage system (also called an underdrain) may be, for example, a grid of drainage pipes covered in gravel or a perforated platform, optionally covered with a layer of gravel. Feed and backwash water troughs cross the tank above the bed of sand. Water fed into the tank from the troughs flows through the sand bed and into the underdrain. The bed is periodically backwashed by feeding water, and optionally compressed air, in through the drainage pipes and collecting backwashed water in the troughs. This type of filter is commonly used in municipal drinking water filtration plants.
Membrane filters use a permeable membrane to filter water. In municipal drinking water plants, the membrane pore size is usually in the ultrafiltration or microfiltration range. In immersed systems, the membrane modules are placed in an open tank and permeate is withdrawn from the inside of the membranes. One commercial product is the ZEEWEED™ 1000 (also called ZW 1000) module sold by GE Water & Process Technologies. These modules are generally as described in U.S. Pat. No. 6,325,928, Immersed Membrane Element and Module, and U.S. Pat. No. 6,893,568, Immersed Membrane Filtration System and Overflow Process, which are incorporated herein by reference. US Patent Application Publication 2006/0108275 A1 describes a kit to integrate an immersed membrane into an existing sand filter and is also incorporated herein by reference.